Preparation of high-strength porous mullite ceramics and the effect of hollow sphere particle size on microstructure and properties

Porous mullite ceramics with high strength were successfully prepared by protein foaming method with fly ash hollow spheres as raw materials. When sintering temperature was increased from 1450 °C to 1550 °C, short rod-like mullite crystals in three-dimensional skeleton grew into interlocking microstructure, effectively enhancing compressive strength and flexural strength of mullite ceramic samples. Using hollow spheres with smaller particle sizes decreased open porosity of samples. However, the effect on total porosity was negligible. Moreover, smaller hollow spheres promoted a shift in median pore size of samples to smaller diameters (87.01 μm→61.66 μm→54.61 μm→36.81 μm). Both compressive strength and flexural strength of samples were significantly improved by reducing particle size of fly ash hollow spheres. Samples produced with the smallest hollow spheres (S4) exhibited low thermal conductivity of 0.401 W m −1 K −1 as well as high compressive strength of 18.10 MPa and flexural strength of 8.69 MPa. These characteristics are significantly superior to those of traditional mullite-based porous ceramics, indicating the effectiveness of the proposed preparation method.